A study of pregnant women and their children in New York City has provided clinical evidence that links environmental pollution with childhood obesity.

The most up-to-date statistics show that 17 per cent of children in the U.S. are obese, and that figure rises to 25 per cent in built-up, inner-city neighborhoods.

While poor diet and lack of exercise are still the major contributors to the national epidemic, this new evidence suggest that air pollution can play a role.

Columbia University's Mailman School of Public Health conducted the study of expecting mothers in New York, and found that those exposed to higher concentrations of airborne chemicals were more than twice as likely to have children who were obese by the age of seven.

The burning of coal, diesel, oil and gas - as well as other substances, such as tobacco - produce chemicals called polycyclic aromatic hydrocarbons (PAH).

The school's report, published in the American Journal of Epidemiology, and its lead author - Dr Andrew Rundle - said:

'Obesity is a complex disease with multiple risk factors. It isn't just the result of individual choices like diet and exercise.'

Getting to grips: The Columbia University study admits that poor diet and low exercise are still the main culprits, but reveal that air pollution also plays a role.

Dr Rundle, a professor of epidemiology, added:

'For many people - who don't have the resources to buy healthy food or don't have the time to exercise - prenatal exposure to air pollution may tip the scales, making them even more susceptible to obesity.'

The women were selected between the ages of 18 and 35, and identified themselves as either African-American or Dominican. They lived in areas in Northern Manhattan or the South Bronx, which are predominantly low-income areas.

Children of women exposed to high levels of PAH during pregnancy were nearly twice as likely (1.79 times) to be obese at age five, and more than twice as likely (2.26 times) to be obese at age seven, compared with children of mothers with lower levels of exposure.

The seven-year-olds whose mothers were in the highest exposure group had, on average, 2.4lb more fat mass than children of mothers with the least exposure.

Previous research from Columbia University found that prenatal exposure to PAH can negatively affect childhood IQs and is linked to anxiety, depression and attention problems in young children.

PAH also disrupt the body's endocrine system and are known carcinogens.

But Dr Rundle said there are ways to reduce PAH exposure.

Certain fuels release more of the chemicals than others, and efforts in New York City to take diesel buses off the streets and retrofit oil furnaces so they burn cleaner fuel was already starting to help.

Comment: Additional information about the negative effects of air pollution on children:

It seems that residents in locations in which air quality levels were below, but close to, acceptable safety limits set forth by the Environmental Protection Agency (EPA), experienced a 20 percent increased incident of being diagnosed with diabetes versus those exposed to few pollutants, said WebMD. Even after factoring known diabetes risks - obesity, race, sedentary living - the strong link existed, wrote WebMD.

Although the study does not definitively link air pollution exposure to diabetes, it does represent the first, large, national study to review the potential connection, noted WebMD. "We know exposure to air pollution is a risk factor for cardiovascular disease," said John Brownstein, PhD, of the Children's Hospital Boston and Harvard Medical School, quoted WebMD. "This is just one more piece of evidence that pollution impacts health," Brownstein added. In studies, chronic inflammation has been linked to insulin resistance, which leads to diabetes; chemicals "found in air pollution have been linked to inflammation," said WebMD.

A study by the Columbia Center for Children's Environmental Health (CCCEH) carried out in Krakow, Poland has found that prenatal exposure to pollutants can adversely affect children's cognitive development at age 5, confirming previous findings in a New York City (NYC) study.

Researchers report that children exposed to high levels of polycyclic aromatic hydrocarbons (PAHs) in Krakow had a significant reduction in scores on a standardized test of reasoning ability and intelligence at age 5. The study findings are published online in Environmental Health Perspectives.

PAHs are released into the air from the burning of fossil fuels for transportation, heating, energy production, and from other combustion sources.